1. Understand the principles and significance of the melting point determination
2. Master the capillary melting point method and the microscopic melting point tester to determine the melting point of the operating method
Second, the basic concepts and principles (add Figure 1. material vapor pressure with temperature curve, 2, phase with time graph
Definition of the melting point: the temperature at which the solid-liquid state of the compound becomes equilibrium under atmospheric pressure, called the melting point of the chemical compound. Solid-liquid two states in atmospheric pressure into equilibrium temperature, said the melting point of the compound.
Melting range: the initial melting to the full melting range called melting range. The melting range of pure substances does not exceed 0.5-1.0 ℃. Substances containing impurities, its melting point is lower than the pure substance, and the melting range is longer.
Meaning: Determination of the melting point is of great value in the identification of organic substances and qualitative judgment of the purity of solid compounds.
Third, the experimental apparatus and reagents used
urea: cinnamic acid (1:1), urea, cinnamic acid, b-shaped tube, thermometer with a notched stopper, the iron stand, melting point of the tube, alcohol lamps, surface dishes, long glass tubes, microscopic melting point apparatus
Fourth, the experimental setup
melting bath: liquid paraffin as a hot bath
Tetra Pak tubes (b-shaped tubes)
a. The melting point is determined by the melting point of a solid compound, and the melting range is longer than that of a pure substance.
a. Mercury ball should be located between the b-shaped tube up and down the fork tube mouth; b. Sample part should be located in the middle of the mercury ball;
c. b-shaped tube to add liquid height up to the fork tube can be; d. Heating in the position shown in the figure, so that the heat is uniform.
IV. Experimental content
1. Determination of the melting point of urea, cinnamic acid and mixed samples by capillary method: rough measurement once, fine measurement twice;
2. Use of microscopic melting point tester to determine the melting point of urea, cinnamic acid.
V. Operating procedures
(a) capillary melting point determination
(1) the preparation of melting point tube: glassblower operation: pulling 1mm in diameter, 7-8cm long, one end of the closed glass tube;
(2) the sample is loaded: the sample should be studied fine, sample height of 2-3mm, the drug loaded to be uniform, strong;
(3) the sample is loaded: the sample should be studied fine, sample height of 2-3mm, drug loaded to be uniform, firm;
(4) After assembling the instrument, heating on low heat;
(4) 10-15 ℃ from the melting point, control the rate of temperature increase of 1-2 ℃ / min;
(5) note down the sample began to collapse and there is a liquid phase (initial melting) and the disappearance of solids completely (full melting) thermometer readings, that is, the melting range of the compound.
(6) Repeat the above operation, and determine separately.
(B) the melting point of the melting point of the melting point tester (demonstration)
Note:
1. \x09 samples used must be dry, fine, packed solid.
2. \x09 should pay attention to observe whether the initial melting before the shrinkage or softening, gas emission and other decomposition phenomena. For example, a substance at 112 ℃ began to shrink, at 113 ℃ droplets appear, at 114 ℃ all liquefied, should be recorded as follows: melting point of 113-114 ℃, 112 ℃ shrinkage.
3. \x09 rough measurement of melting point when the heating rate is slightly faster, to understand the approximate melting point of the sample range.
4. \x09 fine measurement of the heating rate first fast and then slow, to the melting point of 10-15 ℃ from the melting point, control the heating rate of 1-2 ℃ / min.
5. \x09 measurement of another sample, to wait until the hot bath temperature down to about 30 ℃ below its melting point and then measured.
Six, think and discuss
1. \x09 how to use the melting point melting point determination to determine whether A and B are the same substance?
2. \x09 measurement of melting point, if you encounter the following situations, what will be the results?
(1) \x09 melting point tube is not clean.
(2) The bottom of the \x09 melting point tube is not completely closed, there is still a pinhole.
(3) The wall of the \x09 melting point tube is too thick.
(4)\x09 sample is not completely dry or contains impurities.
(5)\x09 heated too quickly.
(6)\x09 sample is not finely ground or packed tightly.